The DECT (Digital Enhanced Cordless Telecommunications) is the protocol for European digital wireless telephone and can be applied for residential PABX (Private Automatic Branch Exchange). So far there is no design for direct communication of two handsets in Europe, which means that two handsets must communicate with each other through the base and be located in a place where the radio signal from the base can be received.
Referring to FIG. 1, there is schematically shown the present condition. The Base 1 is necessary for the communication of the Handset#12 and the Handset#24 which both communicate with the external (by the PSTN/ISDN) through the exchange 3 of the telephone company. The curve BSR is the boundary of the service range of Base 1. We can see that the Handset#125 is out of the service range and cannot communicate with Handset.multidot.24 though their distance is very short. The multiple handsets (parties, branches or extensions) DECT wireless telephone would be more attractive if it possesses a function similar to walkie-talkie as handsets are located far away from where the base is placed.
Referring now to FIG. 2, the DECT wireless communication standard belongs to a TDMA (Time Division Multiple Access) system and is planned to occupy 10 channels (carriers) each of which is called a DECT frame divided into 24 time slots and programmed into 12 Full Duplex Bearers wherein 12 time slots are used for up link communication and another 12 time slots for down link communication. In such DECT system, handsets communicate with each other through base 1, cannot communicate with the external if located outside of the service range of base 1, and cannot communicate with each other directly.
According to the Japanese PHS (Personal Handyphone System) which selects 10 channels out of all available channels for communicating PHS handsets, however, handsets are directly communicable in the following manner. In direct communication mode continuously scanning the 10 channels, when a (calling) handset is to call, an empty channel is selected to emit the control signal to call another (called) handset which will, once receiving the calling signal, answer the calling handset to connect and directly communicate with each other.
Since the 10 channels are to be continuously scanned and the signals are to be continuously emitted which thus are relatively energy-consuming, in order to be energy-saving, some one suggests calling and called handsets can communicate with each other if they transmit and receive signals both in the same one of the time slots of the same one of the 10 channels through the synchronous characteristics of the handset and the base. Such system still suffers from the disadvantage that handsets must be located within a region capable of receiving the radio signal of the same base in order to be able to be sychronous.
It is therefore tried by the applicant to overcome the above problems encountered by the prior art.